1. Techninal Field
The present invention relates to a turbo compound engine that is equipped with a power turbine and an exhaust bypass mechanism; wherein
the power turbine is so designed as to be able, under the control of the exhaust bypass mechanism, to assist the engine in an engine driving mode of operation by recovering the exhaust gas energy as the power turbine is rotating in a normal direction (called "turbine drive" hereinafter) while exerting on the engine a braking force by pumping work the power turbine performs as the power turbine is driven by the engine in the reverse direction as a compressor ("turbine brake"), where the medium that the turbine deals with is exhaust gas or intake air or a mixture of the two; and
the exhaust bypass mechanism, which is composed of a bypass line that lets the exhaust gas bypass the turbine and a bypass valve that controls the opening of the bypass line, is so designed as to determine whether the turbine's operation mode is turbine drive or turbine brake.
2. Background Art
In turbo compound engine systems, it is usual, as shown in FIG. 4, to obtain an additional power output in the engine 2 that is equipped with a turbocharger 1 by recovering the exhaust gas energy, first with the turbocharger 1 by supercharging the intake air into the engine 2, then with a power turbine 4, which is provided in the exhaust path 3 downstream of the turbocharger 1, by generating rotation power and applying it to the crank shaft 5 of the engine 2.
Here, it is to be noted that an engine system as described above, namely, one whose engine 2 is boosted up with the roation power generated by turbine 4 in addition to the intake air supercharging by turbocharger 1, demands a considerable braking force.
For this requirement, an engine system has been developed that is capable of performing an additional engine braking besides performing normal foot-brake braking and exhaust gas braking, which additional braking force is generated by driving an exhaust gas energy recovery turbine 4 from the engine 2 side, not in its normal direction of rotation but in the reverse direction, so that the turbine 4 will perform pumping action as a compressor. An example is shown in FIG. 4, where a reverser 6 to perform rotation reversing of the turbine 4 and a coupling mechanism 7, which consist of reduction gears and a fluid coupling to absorb the rotary inertia (i.e., slipping) as well as adjust the revolution speed of turbine 4 when it is reversed, are provided between the turbine 4 and the crank shaft 5 so as to perform transmission of the driving power in both directions.
In the figure, numeral 8 refers to a hydraulic system to control the reverser 6, and numeral 9 to another hydraulic system to cool the reverser 6 from heat generated upon slipping and other causes during reversing movement.
To this engine system, there is also provided an intake air bypass 11 for letting the turbine 4 utilize the intake air while it is performing pumping work as a compressor as driven by the engine 2 in the turbine brake mode of operation, in addition to the exhaust bypass 3 that feeds the exhaust gas to the turbine 4 (in the direction of arrow A) when operated in the turbine drive mode. The intake air bypass 11 is laid out so as to lead the intake air from the intake path 10 to the delivery side of the turbine 4 (in the direction of arrow B), and it is controlled by a shut-off valve 12 so that the intake air will be fed to the turbine 4 only when it is in the turbine brake mode of operation (i.e., when the rotation of the turbine is reversed).
In the exhaust bypass 3, on the other hand, there is provided a check valve 13 to close up the exhaust bypass 3 while the turbine 4 is reversed, so as to block the back-flow of exhaust gas. Between the turbocharger 1 and the turbine 4, moreover, there is provided an exhaust brake valve 14 that closes up the exhaust bypass 3 when the exhaust gas braking is applied.
Furthermore, this system is equipped with an exhaust bypass mechanism 17, which is composed of a bypass line 15 that bypasses the exhaust gas from turbine 4 (in the direction of arrow C) and a bypass valve 16 that controls the opening of the bypass line 15. Here, the exhaust bypass mechanism 17 works to allow the exhaust gas to bypass the turbine 4 whenever the load on the engine 2 is so low that feeding of the exhaust to turbine 4 may result in unnecessary rise of the back pressure within the exhaust bypass 3, and as a consequence in bringing about undesired power loss in the turbocharger 1 and in the engine 2.
The functions of the bypass valve 16 are as follows: when the engine load, as indicated by a signal the engine 2 gives out, for example, by the position of the control rack of a governor (this will be called simply "rack" hereinafter), is within the predetermined load range and, at the same time, the engine revolution speed is within the predetermined revolution speed range (the domain in which the engine is driven by the turbine, D (called "turbine" drive domain D" hereinafter)), the bypass valve 16 is closed to stop the bypassing of exhaust gas as shown in FIG. 5, while feeding the turbine 4 with exhaust gas to allow the turbine 4 to drive the engine 2; whereas when the engine load situation is outside domain D (the domain in which the exhaust gas is bypassed, E ("exhaust bypass domain E")), the bypass valve 16 is opened to allow the exhaust gas to bypass the turbine 4.
These relations are illustrated in FIG. 6. Namely, in the domains D and E of normal operation of the engine 2 (i.e., everywhere excepting the domain of turbine-assisted engine brake mode of operation F (the "turbine brake domain F"), the exhaust bypassing is activated or deactivated in accordance with whether the operation is within the range defined by the predetermined engine revolutions and predetermined engine loads or without it (in the turbine drive domain D or in the exhaust bypass domain E).
Referring now to FIG. 7, in an engine system as described above, signals coming from the key switch 31, an accelerator switch 18, and a clutch switch 19, the exhaust brake switch 20, the turbine brake switch 21, the rack sensor 22, and the engine revolution sensor 23 are input to a CPU 28 of the control unit 24 through the signal input circuit 25, an A/D converter 26, and a waveform shaping circuit 27. Then the signals are processed in the CPU 28 with reference to the control conditions stored in ROM 29, such as the operation map illustrated in FIG. 6, so that the control signals are sent out to the reverser 6 and valves 12 to 14 and 16 from the output circuit 30. Here, it is to be noted that beside the operation described above, i.e., storing of entire references in a ROM, other operations, for example, with use of RAM for instantaneous and progressive data precessing are equally satisfatory.
In the aforedescribed exhaust bypass control, in particular, the rack sensor 22, the engine revolution sensor 23, and the turbine brake switch 21 participate cooperatively in a manner such that when the turbine brake switch 21 is turned on, the operation is shifted from the two domains of normal engine operation mode, D and E (the "normal engine operation domain D, E") to the turbine brake domain F (shifting in the opposite direction being executed when the turbine brake switch 21 is turned off), while the changeover between the two domains of normal engine drive operation mode D and E is conducted with reference to the values of signals coming from the rack sensor 22 and the engine revolution sensor 23.
Now, the control of valves 12 to 14 and 16, particularly that of the bypass valve 16 of the exhaust bypass mechanism 17, is conducted in a flow chart described below (see FIGS. 8 and 9):
After initializing the control unit 24 with initial data, the control unit 24 reads in the signals that have been mentioned in the foregoing paragraphs and processes them. Namely, in the normal operation range, firstly the exhaust bypass control is executed on the basis of engine load and engine revolution speed once engagement of the clutch is confirmed by the clutch switch 19.
In this case, the shut-off valve 12 is always closed and the exhaust brake valve 14 is always opened, so that when the turbine 4 is in the normal turbine drive mode (domain D), the exhaust path 3 is opened (the check valve 13 is open) while the bypass line 15 is closed (the bypass valve 16 is closed) to feed the exhaust to the turbine 4. When the operation is in the exhaust bypass domain E, on the other hand, it is preferable to close the exhaust path 3, even though it may be left open (the check valve 13 is either closed or open), while opening the bypass line 15 (the bypass valve 16 is open), so that the exhaust gas will flow bypassing the turbine 4.
Subsequently, as the turbine brake switch 21 is turned on, the turbine 4 is reversed, and the operation is shifted over to the turbine brake domain F. In this case, the control of valves 12 to 14 and 16 is such that the shut-off valve 12 of the intake air bypass 11 and the bypass valve 16 of the bypass line 15 are both opened so as to drive the turbine 4 as a compressor, while the check valve 13 of the exhaust path 3 is closed. The exhaust brake valve 14 is closed/opened as the exhaust brake switch 20 is turned on/off. These states of operation are returned to the normal exhaust bypass mode of controlling described above as the turbine brake switch 21 is turned off.
Here, the prior practice has been such that, when the engine operation is shifted from the turbine brake domain F over to the turbine drive domain D, the control is so set as to let the operation go through the exhaust bypass domain E after exiting the domain F and before entering the domain D, as shown by arrows H in FIG. 6.
Such a control suffers from following problems: for example, when it is desired to accelerate the engine 2 within a relatively short period of time from the state of the turbine brake, the acceleration is recognized first by the rack sensor 22 as an sharp increase in the engine load and by the engine revolution sensor 23 as a sharp increase in the engine revolution, as shown by arrow J in FIG. 6. Here, the difficulty is associated with controlling of the bypass valve 16. That is to say, because of the necessity of going through the exhaust bypass domain E as shown in FIG. 6, the bypass valve 16 is left open, keeping the bypass line 15 opened for a considerable period of time.
Now, it is true that the bypass valve 16 starts closing toward the final stage in the exhaust bypass domain E, raising the pressure of the exhaust path 3 and increasing revolution of the turbine 4 as shown in FIG. 3, but until such a state of affairs has been attained, the pressure within the exhaust path 3 cannot become sufficiently high, even though a large quantity of fuel is being fed into the engine 2 obeying the command of acceleration and raising the pressure of the exhaust gas, because the increase in the pressure of the exhaust path 3 is dissipated through the bypass line 15 (as shown by G in the figure).
This results in "breathing" during the rise in the power turbine revolution speed, making the output increase stepwise rather than smooth, and degrading the response characteristics.
To summarize, the central problem in this way of controlling has been as follows:
as the engine operation mode is shifted from turbine brake mode to normal mode, the bypass valve 16 has to be kept open in the exhaust bypass domain E, so that smooth increase of the turbine revolution in accordance with the exhaust gas temperature and the rise in the exhaust gas pressure cannot be attained.